Recently, an article, such as a film, which has a concave-convex microstructure with a period that is equal to or less than the wavelength of visible light on a surface thereof has been known to exhibit an antireflection effect, a lotus effect, and the like. In particular, a concave-convex structure referred to as a moth-eye structure is known to serve as an effective antireflection means since a refractive index continuously increases from the refractive index of air to the refractive index of the material of the article.
As a method of forming a concave-convex microstructure on a surface of an article, attention is drawn to a method in which a mold having an inverted structure of the concave-convex microstructure formed on the surface thereof is used, a curable resin composition is supplied between the mold and an article main body (substrate) and cured, and the concave-convex microstructure of the mold is transferred to the surface of the article main body (nanoimprinting method).
As a method for producing a mold for nanoimprinting, a method is known in which a step of forming an oxide film having fine pores by anodizing an aluminum substrate in an electrolyte and a step of increasing the fine pores of the oxide film by etching are alternately repeated to form anodized alumina having a plurality of fine pores (concave portions) on the surface of the aluminum substrate.
However, the surface of the aluminum substrate may be mirror-polished by mechanical processing such as cutting processing in some cases.
However, when the machined aluminum substrate is anodized, an oxide film having fine pores non-uniformly dispersed is easily formed. When a mold in a state where fine pores are non-uniformly dispersed is used to transfer the concave-convex microstructure to the surface of the article main body, haze easily increases, and as a result, reflectance also increases.
The reason why the oxide film having fine pores non-uniformly dispersed is formed is considered that an affected layer is formed on the surface of the aluminum substrate by mechanical processing.
As a method for producing a mold in which fine pores are uniformly dispersed even when the machined aluminum substrate is anodized, for example, Patent Document 1 discloses a method of performing cathode electrolysis, electrolytic polishing, or etching on an aluminum substrate before an anodizing step. In the method described in Patent Document 1, a fine concave-convex structure is formed on an affected layer by cathode electrolysis, electrolytic polishing, or etching, and an oxide film having fine pores more uniformly dispersed is formed using these concave portions by anodization. Further, in Patent Document 1, after the affected layer is removed by cathode electrolysis, electrolytic polishing, or etching, an oxide film can also be formed.